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Date: Sat, 10 Sep 2016 18:37:29 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Christoph Hellwig <hch@infradead.org>, Todd Kjos <tkjos@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Arve Hj??nnev??g <arve@android.com>,
        Riley Andrews <riandrews@android.com>, devel@driverdev.osuosl.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] android: binder: Disable preemption while holding the
 global binder lock
In-Reply-To: <20160910162210.GK10153@twins.programming.kicks-ass.net>
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References: <CAHRSSEwkj-VxstqCm6JzVQcmZzxLQcob0JNiEhzes9oT0-tXSg@mail.gmail.com> <20160910161659.GA7987@infradead.org> <20160910162210.GK10153@twins.programming.kicks-ass.net>
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On Sat, 10 Sep 2016, Peter Zijlstra wrote:

> On Sat, Sep 10, 2016 at 09:16:59AM -0700, Christoph Hellwig wrote:
> > On Thu, Sep 08, 2016 at 09:12:50AM -0700, Todd Kjos wrote:
> > > In Android systems, the display pipeline relies on low
> > > latency binder transactions and is therefore sensitive to
> > > delays caused by contention for the global binder lock.
> > > Jank is siginificantly reduced by disabling preemption
> > > while the global binder lock is held.
> > 
> > That's now how preempt_disable is supposed to use.  It is for critical
> 
> not, that's supposed to be _not_. Just to be absolutely clear, this is
> NOT how you're supposed to use preempt_disable().
> 
> > sections that use per-cpu or similar resources.
> > 
> > > 
> > > Originally-from: Riley Andrews <riandrews@google.com>
> > > Signed-off-by: Todd Kjos <tkjos@google.com>
> 
> > > @@ -389,7 +390,11 @@ static int task_get_unused_fd_flags(struct
> > > binder_proc *proc, int flags)
> > >   rlim_cur = task_rlimit(proc->tsk, RLIMIT_NOFILE);
> > >   unlock_task_sighand(proc->tsk, &irqs);
> > > 
> > > - return __alloc_fd(files, 0, rlim_cur, flags);
> > > + preempt_enable_no_resched();
> > > + ret = __alloc_fd(files, 0, rlim_cur, flags);
> > > + preempt_disable();
> 
> And the fact that people want to use preempt_enable_no_resched() shows
> that they're absolutely clueless.
> 
> This is so broken its not funny.
> 
> NAK NAK NAK

Indeed. Sprinkling random preempt_enabe/disable() pairs all over the place
documents clearly that this is tinkering and not proper software
engineering.

NAK from my side as well.

Thanks,

	Thomas